1. Field of the Invention
The present invention relates to a portable radio communication device for digital cellular communication, and more particularly to a radio communication device capable of communication with other digital cellular communication systems.
2. Description of the Related Art
As various communication systems are available in recent years, there have been proposed communication devices that are compatible with different communication systems. For example, a communication device disclosed in Japanese laid-open patent publication No. 6-21880 has a main communication unit which operates with a cellular communication system or a digital cellular communication system, and which can be made compatible with another cellular communication system or another digital cellular communication system by the addition of an auxiliary communication unit.
Japanese laid-open patent publication No. 5-252107 reveals a communication device that operates as an analog mobile telephone set by itself and, when combined with an extended-service radio unit, can operate as a digital mobile telephone set capable of transmitting and receiving digital radio signals.
The disclosed communication devices are composed of either a main communication unit with an added auxiliary communication unit or an analog mobile telephone set with an added extended-service radio unit. These prior communication devices are made compatible with different communication systems by connecting independently operable units to each other, and their compatible circuit arrangement includes independent systems that are positioned between a certain point in a signal system which is continuously connected and an antenna. Even though the compatible circuit arrangement includes shared components, those independent systems are composed of independent components that are common, but not shared by the systems. As a result, the compatible circuit arrangement has not been economical as it has two sets of common components respectively in the independent systems.
If the common components were to be shared by the independent systems, then the number and length of signal paths through the common components between the independent systems would be so large that the communication devices would have their characteristics degraded particularly when handling high-frequency signals. Recent communication devices which have a hybrid arrangement of components would be made too complicated by a shared use of such common components between independent systems thereof.
Therefore, communication devices that are compatible with different communication systems are preferably provided with systems dedicated to those different communication systems for economic and characteristic reasons. However, some digital communication systems, e.g., some digital cellular communication systems, differ from each other only with respect to their frequencies, output powers, and control protocols. Adding dedicated systems to make communication devices compatible with those digital communication systems is not economically preferable. Therefore, it has been desirable to make efforts to share certain common components between systems which provide compatibility with those digital communication systems.